GEORGE CLARK/Associated PressDallas linebacker Anthony Spencer kneels over New Orleans QB Drew Brees after a sack in the Cowboys' 24-17 victory on Saturday night.NEW ORLEANS -- Ever wondered why people still talk about the 1972 Miami Dolphins? The New Orleans Saints offered a convincing answer Saturday night in the Superdome.

The '72 Dolphins remain the only NFL team in the Super Bowl era to go through a complete season undefeated. The 2007 New England Patriots came the closest to joining them, finishing the regular season 16-0 and advancing to the Super Bowl, where they were upset by the Giants.

Now that the 2009 Saints' stab at perfection has ended with a 24-17 loss to Dallas, it's easy to see why the '72 Dolphins have stood alone for so many years.

It's tough to bring the same level of intensity every week in the NFL, to play every game like it means the world when a playoff berth is already assured, a first-round bye clinched. It's almost too much to ask to win them all in a league where injuries are the norm and a short injured reserve list is practically unheard of.

Indianapolis, which improved to 14-0 with a 35-31 win over Jacksonville on Thursday, still has a shot at perfection. But could giving up 31 points to a Jaguars team that's averaging just 19 be seen as a hint of weakness, a sign of things to come?

Sooner or later, the rigors of a 16-game regular season and an expanded postseason have caught up with everybody. It didn't catch up with the 2007 Patriots until the Super Bowl, but there were signs it was coming. They weren't their usual dominant selves in playoff wins over Jacksonville and San Diego before New York popped them.

The Saints were teetering before Saturday night's loss. They haven't played well since whipping the Patriots on "Monday Night Football" late last month, squeaking out road wins over a 4-9 Washington team and a 6-7 Atlanta squad that was playing without its starting quarterback and leading rusher.

Dallas had no such issues and wasted no time ripping into the Saints, as New Orleans' post-Patriots hangover stretched into a third week. The Cowboys outgained New Orleans 195 to 35 in the first quarter, racing out to a 14-0 lead that the Saints never could overcome despite a game fourth-quarter rally.

Drew Brees did not have one of his better nights, despite 298 yards. The offensive line was beaten for four sacks, including a fourth-quarter forced fumble by Demarcus Ware that sealed the win for the Cowboys. The running game produced only 65 yards. The secondary, riddled by injuries throughout the second half of the season, finally gave way.

While it's discouraging the Saints won't be going undefeated, what's worse is that they may be fading down the stretch, that they may have already played their best football. The 2009 season will not be defined by what New Orleans did up to this point, but what it does from here on out.

If the Saints go on to win the Super Bowl, few will care that they lost to Dallas. It'll be a footnote in a historic season, much like the Chicago Bears' lone loss in 1985.

But if the Saints stumble in the playoffs, the win over the Patriots will be remembered as the high-water mark of the season. And Saturday night's loss will be remembered as the beginning of the end of New Orleans' dream season.